1. Field of the Invention
The present invention relates to the field of electrical circuits. More particularly, the invention relates to amplifier circuits.
2. Background Art
Power amplifiers, such as saturated power amplifiers, used in communications devices, such as mobile telephones, are required to operate under wide variations in load impedance. Under these variations in load impedance, it is highly desirable for power amplifiers, such as saturated power amplifiers, to maintain a constant output power. It is also highly desirable for power amplifiers in transmitter systems utilizing predistortion for complex data modulations, such as data modulations utilized in the Enhanced Data Rates for GSM Evolution (EDGE) communications standard, to maintain a constant output power under load impedance variations. As a result, various techniques have been employed in an attempt to control the output power of a power amplifier.
However, conventional techniques for controlling the output power of a power amplifier suffer various disadvantages. For example, techniques that indirectly control the output power by controlling the voltage or current, such as collector voltage or collector current, supplied to the power amplifier provide adequate compensation for variations in operating conditions, such as variations in temperature and supply voltage, but undesirably allow significant variations in power delivered to a varying load. For example, in the voltage or current control techniques discussed above, the power delivered to the load by the power amplifier can vary by as much as 10.0 decibels (dB) as the impedance of the load changes.
In another conventional approach, a constant DC power is delivered to a power amplifier, such as a saturated power amplifier, by utilizing a feedback loop to adjust the collector voltage of the power amplifier. Since the power amplifier output power is equal to the product of the DC power and the collector efficiency, this approach can provide a constant output power if constant collector efficiency is maintained. However, this approach presents significant problems in transmitter systems utilizing predistortion, such as transmitter systems utilizing polar EDGE modulation.